Cellular senescence and chromatin structure

Aging and Lifestyle Modifications for Preventing Aging-Related Diseases

The pathogenesis of various chronic diseases is closely associated with aging. Aging of the cardiovascular system promotes the development of severe cardiovascular diseases with high mortality, including atherosclerosis, coronary heart disease, and myocardial infarction. Similarly, aging of the nervous system promotes the development of neurodegenerative diseases, such as Alzheimer's disease, which seriously impairs cognitive function. Aging of the musculoskeletal system is characterized by decreased function and mobility. The molecular basis of organ aging is cellular senescence, which involves multiple cellular and molecular mechanisms, such as impaired autophagy, metabolic imbalance, oxidative stress, and persistent inflammation. Given the ongoing demographic shift toward an aging society, strategies to delay or reduce the effects of aging have gained significance. Lifestyle modifications, such as exercise and calorie restriction, are now recognized for their anti-aging effects, their capacity to reduce modification, their potential to prolong lifespan, and their capacity to lower the risk of cardiovascular disease. This review elucidates the molecular mechanisms and application significance of various anti-aging approaches at the molecular level, based on research progress in aging. It aims to provide a reference for the prevention and treatment of age-related diseases in progressively aging societies.

The NDR family of kinases: essential regulators of aging

Aging is defined as a progressive decline of cognitive and physiological functions over lifetime. Since the definition of the nine hallmarks of aging in 2013 by López-Otin, numerous studies have attempted to identify the main regulators and contributors in the aging process. One interesting group of proteins whose participation has been implicated in several aging hallmarks are the nuclear DBF2-related (NDR) family of serine-threonine AGC kinases. They are one of the core components of the Hippo signaling pathway and include NDR1, NDR2, LATS1 and LATS2 in mammals, along with its highly conserved metazoan orthologs; Trc in Drosophila melanogaster, SAX-1 in Caenorhabditis elegans, CBK1, DBF20 in Saccharomyces cerevisiae and orb6 in Saccharomyces pombe. These kinases have been independently linked to the regulation of widely diverse cellular processes disrupted during aging such as the cell cycle progression, transcription, intercellular communication, nutrient homeostasis, autophagy, apoptosis, and stem cell differentiation. However, a comprehensive overview of the state-of-the-art knowledge regarding the post-translational modifications of and by NDR kinases in aging has not been conducted. In this review, we summarize the current understanding of the NDR family of kinases, focusing on their relevance to various aging hallmarks, and emphasize the growing body of evidence that suggests NDR kinases are essential regulators of aging across species.

Senescence in cancer: Advances in detection and treatment modalities

Senescence is a form of irreversible cell cycle arrest. Senescence plays a dual role in cancer, as both a tumor suppressor by preventing the growth of damaged cells and a cancer promoter by creating an inflammatory milieu. Stress-induced premature senescence (SIPS) and replicative senescence are the two major sub-types of senescence. Senescence plays a dual role in cancer, depending on the context and kind of senescence involved. SIPS can cause cancer by nurturing an inflammatory environment, whereas replicative senescence may prevent cancer. Major pathways that are involved in senescence are the p53-p21, p16INK4A-Rb pathway along with mTOR, MAPK, and PI3K pathways. The lack of universal senescence markers makes it difficult to identify senescent cells in vivo. A combination of reliable detection methods of senescent cells in vivo is of utmost importance and will help in early detection and open new avenues for future treatment. New strategies that are being developed in order to tackle these shortcomings are in the field of fluorescent probes, nanoparticles, positron emission tomography probes, biosensors, and the detection of cell-free DNA from liquid biopsies. Along with detection, eradication of these senescent cells is also important to prevent cancer reoccurrence. Recently, the field of nano-senolytic and immunotherapy has also been emerging. This review provides up-to-date information on the various types of advancements made in the field of detection and treatment modalities for senescent cells that hold promise for the future treatment and prognosis of cancer, as well as their limitations and potential solutions.

Connecting the dots: Neuronal senescence, stress granules, and neurodegeneration

Cellular senescence increases with aging. While senescence is associated with an exit of the cell cycle, there is ample evidence that post-mitotic cells including neurons can undergo senescence as the brain ages, and that senescence likely contributes significantly to the progression of neurodegenerative diseases (ND) such as Alzheimer's Disease (AD) and Amyotrophic Lateral Sclerosis (ALS). Stress granules (SGs) are stress-induced cytoplasmic biomolecular condensates of RNA and proteins, which have been linked to the development of AD and ALS. The SG seeding hypothesis of NDs proposes that chronic stress in aging neurons results in static SGs that progress into pathological aggregates Alterations in SG dynamics have also been linked to senescence, though studies that link SGs and senescence in the context of NDs and the aging brain have not yet been performed. In this Review, we summarize the literature on senescence, and explore the contribution of senescence to the aging brain. We describe senescence phenotypes in aging neurons and glia, and their links to neuroinflammation and the development of AD and ALS. We further examine the relationships of SGs to senescence and to ND. We propose a new hypothesis that neuronal senescence may contribute to the mechanism of SG seeding in ND by altering SG dynamics in aged cells, thereby providing additional aggregation opportunities within aged neurons.

Multiple myeloma, a quintessential malignant disease of aging: a geroscience perspective on pathogenesis and treatment

Multiple myeloma (MM) is an incurable plasma cell malignancy, which is predominantly a disease of older adults (the median age at diagnosis is 70 years). The slow progression from asymptomatic stages and the late-onset of MM suggest fundamental differences compared to many other hematopoietic system-related malignancies. The concept discussed in this review is that age-related changes at the level of terminally differentiated plasma cells act as the main risk factors for the development of MM. Epigenetic and genetic changes that characterize both MM development and normal aging are highlighted. The relationships between cellular aging processes, genetic mosaicism in plasma cells, and risk for MM and the stochastic processes contributing to clonal selection and expansion of mutated plasma cells are investigated. In line with the DNA damage accumulation theory of aging, in this review, the evolution of monoclonal gammopathy to symptomatic MM is considered. Therapeutic consequences of age-dependent comorbidities that lead to frailty and have fundamental influence on treatment outcome are described. The importance of considering geriatric states when planning the life-long treatment course of an elderly MM patient in order to achieve maximal therapeutic benefit is emphasized.

Senolytic Therapy: A Potential Approach for the Elimination of Oncogene-Induced Senescent HPV-Positive Cells

Senescence represents a unique cellular stress response characterized by a stable growth arrest, macromolecular alterations, and wide spectrum changes in gene expression. Classically, senescence is the end-product of progressive telomeric attrition resulting from the repetitive division of somatic cells. In addition, senescent cells accumulate in premalignant lesions, in part, as a product of oncogene hyperactivation, reflecting one element of the tumor suppressive function of senescence. Oncogenic processes that induce senescence include overexpression/hyperactivation of H-Ras, B-Raf, and cyclin E as well as inactivation of PTEN. Oncogenic viruses, such as Human Papilloma Virus (HPV), have also been shown to induce senescence. High-risk strains of HPV drive the immortalization, and hence transformation, of cervical epithelial cells via several mechanisms, but primarily via deregulation of the cell cycle, and possibly, by facilitating escape from senescence. Despite the wide and successful utilization of HPV vaccines in reducing the incidence of cervical cancer, this measure is not effective in preventing cancer development in individuals already positive for HPV. Accordingly, in this commentary, we focus on the potential contribution of oncogene and HPV-induced senescence (OIS) in cervical cancer. We further consider the potential utility of senolytic agents for the elimination of HPV-harboring senescent cells as a strategy for reducing HPV-driven transformation and the risk of cervical cancer development.

Pancreatic Cancer and Cellular Senescence: Tumor Microenvironment under the Spotlight

Pancreatic ductal adenocarcinoma (PDAC) has one of the most dismal prognoses of all cancers due to its late manifestation and resistance to current therapies. Accumulating evidence has suggested that the malignant behavior of this cancer is mainly influenced by the associated strongly immunosuppressive, desmoplastic microenvironment and by the relatively low mutational burden. PDAC develops and progresses through a multi-step process. Early in tumorigenesis, cancer cells must evade the effects of cellular senescence, which slows proliferation and promotes the immune-mediated elimination of pre-malignant cells. The role of senescence as a tumor suppressor has been well-established; however, recent evidence has revealed novel pro-tumorigenic paracrine functions of senescent cells towards their microenvironment. Understanding the interactions between tumors and their microenvironment is a growing research field, with evidence having been provided that non-tumoral cells composing the tumor microenvironment (TME) influence tumor proliferation, metabolism, cell death, and therapeutic resistance. Simultaneously, cancer cells shape a tumor-supportive and immunosuppressive environment, influencing both non-tumoral neighboring and distant cells. The overall intention of this review is to provide an overview of the interplay that occurs between senescent and non-senescent cell types and to describe how such interplay may have an impact on PDAC progression. Specifically, the effects and the molecular changes occurring in non-cancerous cells during senescence, and how these may contribute to a tumor-permissive microenvironment, will be discussed. Finally, senescence targeting strategies will be briefly introduced, highlighting their potential in the treatment of PDAC.

Hydrogen sulfide in longevity and pathologies: Inconsistency is malodorous

Hydrogen sulfide (H₂S) is one of the biologically active gases (gasotransmitters), which plays an important role in various physiological processes and aging. Its production in the course of methionine and cysteine catabolism and its degradation are finely balanced, and impairment of H₂S homeostasis is associated with various pathologies. Despite the strong geroprotective action of exogenous H₂S in C. elegans, there are controversial effects of hydrogen sulfide and its donors on longevity in other models, as well as on stress resistance, age-related pathologies and aging processes, including regulation of senescence-associated secretory phenotype (SASP) and senescent cell anti-apoptotic pathways (SCAPs). Here we discuss that the translation potential of H₂S as a geroprotective compound is influenced by a multiplicity of its molecular targets, pleiotropic biological effects, and the overlapping ranges of toxic and beneficial doses. We also consider the challenges of the targeted delivery of H₂S at the required dose. Along with this, the complexity of determining the natural levels of H₂S in animal and human organs and their ambiguous correlations with longevity are reviewed.

A Two-Photon Probe Based on Naphthalimide-Styrene Fluorophore for the In Vivo Tracking of Cellular Senescence

Cellular senescence is a state of stable cell cycle arrest that can negatively affect the regenerative capacities of tissues and can contribute to inflammation and the progression of various aging-related diseases. Advances in the in vivo detection of cellular senescence are still crucial to monitor the action of senolytic drugs and to assess the early onset or accumulation of senescent cells. Here, we describe a naphthalimide-styrene-based probe (HeckGal) for the detection of cellular senescence both in vitro and in vivo. HeckGal is hydrolyzed by the increased lysosomal β-galactosidase activity of senescent cells, resulting in fluorescence emission. The probe was validated in vitro using normal human fibroblasts and various cancer cell lines undergoing senescence induced by different stress stimuli. Remarkably, HeckGal was also validated in vivo in an orthotopic breast cancer mouse model treated with senescence-inducing chemotherapy and in a renal fibrosis mouse model. In all cases, HeckGal allowed the unambiguous detection of senescence in vitro as well as in tissues and tumors in vivo. This work is expected to provide a potential technology for senescence detection in aged or damaged tissues.

Cell Senescence, DNA Damage, and Metabolism

Significance: Senescence is an essential biological process that blocks tumorigenesis, limits tissue damage, and aids embryonic development. However, once senescent cells accumulate in tissues during aging, they promote the development of age-related diseases and limit health span. Thus, it is essential to expand the boundaries of our knowledge about the mechanisms responsible for controlling cellular senescence. Recent Advances: Cellular metabolism plays a significant role in the regulation of various signaling processes involved in cell senescence. In the past decade, our knowledge about the interplay between cell signaling, cell metabolism, and cellular senescence has significantly expanded. Critical Issues: In this study, we review metabolic pathways in senescent cells and the impact of these pathways on the response to DNA damage and the senescence-associated secretory phenotype. Future Directions: Future research should elucidate metabolic mechanisms that promote specific alterations in senescent cell phenotype, with a final goal of developing a new therapeutic strategy. Antioxid. Redox Signal. 34, 324-334.

Effects of wound dressings containing silver on skin and immune cells

Wound dressings with silver have been shown to be cytotoxic in vitro. However, the extrapolation of this cytotoxicity to clinical settings is unclear. We applied dressings with various forms of silver on porcine skin ex vivo and investigated silver penetration and DNA damage. We assessed antimicrobial efficacy, cytotoxicity to skin cells, and immune response induced by the dressings. All dressings elevated the DNA damage marker γ-H₂AX and the expression of stress-related genes in explanted skin relative to control. This corresponded with the amount of silver in the skin. The dressings reduced viability, induced oxidative stress and DNA damage in skin cells, and induced the production of pro-inflammatory IL-6 by monocytes. The oxidative burst and viability of activated neutrophils decreased. The amount of silver released into the culture medium varied among the dressings and correlated with in vitro toxicity. However, antimicrobial efficiencies did not correlate strongly with the amount of silver released from the dressings. Antimicrobial efficiency and toxicity are driven by the form of silver and the construction of dressings and not only by the silver concentration. The damaging effects of silver dressings in ex vivo skin highlight the importance of thorough in vivo investigation of silver dressing toxicity.

Comparison of senescence-related changes between three- and two-dimensional cultured adipose-derived mesenchymal stem cells

BACKGROUND

Adipose-derived mesenchymal stem cells (ADMSCs) have attracted widespread interest as cell-based tissue repair systems. To obtain adequate quantities of ADMSCs for therapeutic applications, extensive in vitro expansion is required. However, under current two-dimensional (2D) approaches, ADMSCs rapidly undergo replicative senescence, and cell growth is impeded and stem cell properties are eliminated by mechanisms that are poorly understood. These issues limit the extensive applications of ADMSCs. In this study, we investigated senescence-related changes in mesenchymal stem cells (MSCs) isolated from human adipose tissue in 2D and three-dimensional (3D) cultures.

METHODS

We studied cell growth over a given period (21 days) to determine if modes of culture were associated with ADMSC senescence. ADMSCs were isolated from healthy females by liposuction surgery and then were grown in 2D and 3D cultures. The cell morphology was observed during cell culture. Every other time of culture, senescence-associated β-galactosidase (SA-β-gal) expression, cell viability, proliferation, and differentiation potential of ADMSCs from 2D and 3D cultures were detected. Also, senescence- and stemness-related gene expression, telomere length, telomerase activity, and energy metabolism of ADMSCs for different culture times were evaluated.

RESULTS

With long-term propagation, we observed significant changes in cell morphology, proliferation, differentiation abilities, and energy metabolism, which were associated with increases in SA-β-gal activity and decreases in telomere length and telomerase activity. Notably, when cultured in 3D, these changes were improved.

CONCLUSIONS

Our results indicate that 3D culture is able to ameliorate senescence-related changes in ADMSCs.

Hypoxia-mediated changes in bone marrow microenvironment in breast cancer dormancy

Breast cancer (BC) remains a clinical challenge despite improved treatments and public awareness to ensure early diagnosis. A major issue is the ability of BC cells (BCCs) to survive as dormant cancer cells in the bone marrow (BM), resulting in the cancer surviving for decades with the potential to resurge as metastatic cancer. The experimental evidence indicates similarity between dormant BCCs and other stem cells, resulting in the preponderance of data to show dormant BCCs being cancer stem cells (CSCs). The BM niche and their secretome support BCC dormancy. Lacking in the literature is a comprehensive research to describe how the hypoxic environment within the BM may influence the behavior of BCCs. This information is relevant to understand the prognosis of BC in young and aged individuals whose oxygen levels differ in BM. This review discusses the changing information on vascularity in different regions of the BM and the impact on endogenous hematopoietic stem cells (HSCs). This review highlights the necessary information to provide insights on vascularity of different BM regions on the behavior of BCCs, in particular a dormant phase. For instance, how the transcription factor HIF1-α (hypoxia-inducible factor 1 alpha), functioning as first responder under hypoxic conditions, affects the expression of specific gene networks involved in energy metabolism, cell survival, tumor invasion and angiogenesis. This enables cell fate transition and facilitates tumor heterogeneity, which in turn favors tumor progression and resistance to anticancer treatments Thus, HIF1-α could be a potential target for cancer treatment. This review describes epigenetic mechanisms involved in hypoxic responses during cancer dormancy in the bone marrow. The varied hypoxic environment in the BM is relevant to understand the complex process of the aging bone marrow for insights on breast cancer outcome between the young and aged.

Nucleus size and DNA accessibility are linked to the regulation of paraspeckle formation in cellular differentiation

BACKGROUND

Many long noncoding RNAs (lncRNAs) have been implicated in general and cell type-specific molecular regulation. Here, we asked what underlies the fundamental basis for the seemingly random appearance of nuclear lncRNA condensates in cells, and we sought compounds that can promote the disintegration of lncRNA condensates in vivo.

RESULTS

As a basis for comparing lncRNAs and cellular properties among different cell types, we screened lncRNAs in human pluripotent stem cells (hPSCs) that were differentiated to an atlas of cell lineages. We found that paraspeckles, which form by aggregation of the lncRNA NEAT1, are scaled by the size of the nucleus, and that small DNA-binding molecules promote the disintegration of paraspeckles and other lncRNA condensates. Furthermore, we found that paraspeckles regulate the differentiation of hPSCs.

CONCLUSIONS

Positive correlation between the size of the nucleus and the number of paraspeckles exist in numerous types of human cells. The tethering and structure of paraspeckles, as well as other lncRNAs, to the genome can be disrupted by small molecules that intercalate in DNA. The structure-function relationship of lncRNAs that regulates stem cell differentiation is likely to be determined by the dynamics of nucleus size and binding site accessibility.

Maintenance of Proteostasis by P Body-Mediated Regulation of eIF4E Availability during Aging in Caenorhabditis elegans

Aging is accompanied by a pervasive collapse of proteostasis, while reducing general protein synthesis promotes longevity across taxa. Here, we show that the eIF4E isoform IFE-2 is increasingly sequestered in mRNA processing (P) bodies during aging and upon stress in Caenorhabditis elegans. Loss of the enhancer of mRNA decapping EDC-3 causes further entrapment of IFE-2 in P bodies and lowers protein synthesis rates in somatic tissues. Animals lacking EDC-3 are long lived and stress resistant, congruent with IFE-2-deficient mutants. Notably, neuron-specific expression of EDC-3 is sufficient to reverse lifespan extension, while sequestration of IFE-2 in neuronal P bodies counteracts age-related neuronal decline. The effects of mRNA decapping deficiency on stress resistance and longevity are orchestrated by a multimodal stress response involving the transcription factor SKN-1, which mediates lifespan extension upon reduced protein synthesis. Our findings elucidate a mechanism of proteostasis control during aging through P body-mediated regulation of protein synthesis in the soma.

Dephosphorylation of p53 Ser 392 Enhances Trimethylation of Histone H3 Lys 9 via SUV39h1 Stabilization in CK2 Downregulation-Mediated Senescence

Cellular senescence is an irreversible form of cell cycle arrest. Senescent cells have a unique gene expression profile that is frequently accompanied by senescence-associated heterochromatic foci (SAHFs). Protein kinase CK2 (CK2) downregulation can induce trimethylation of histone H3 Lys 9 (H3K9me3) and SAHFs formation by activating SUV39h1. Here, we present evidence that the PI3K-AKTmTOR-reactive oxygen species-p53 pathway is necessary for CK2 downregulation-mediated H3K9me3 and SAHFs formation. CK2 downregulation promotes SUV39h1 stability by inhibiting its proteasomal degradation in a p53dependent manner. Moreover, the dephosphorylation status of Ser 392 on p53, a possible CK2 target site, enhances the nuclear import and subsequent stabilization of SUV39h1 by inhibiting the interactions between p53, MDM2, and SUV39h1. Furthermore, p21Cip1/WAF1 is required for CK2 downregulation-mediated H3K9me3, and dephosphorylation of Ser 392 on p53 is important for efficient transcription of p21Cip1/WAF1. Taken together, these results suggest that CK2 downregulation induces dephosphorylation of Ser 392 on p53, which subsequently increases the stability of SUV39h1 and the expression of p21Cip1/WAF1, leading to H3K9me3 and SAHFs formation.

Delay of cell growth and loss of stemness by inhibition of reverse transcription in human mesenchymal stem cells derived from dental tissue

The present study investigated the cellular properties in the dental tissue-derived mesenchymal stem cells (DSCs) exposed to nevirapine (NVP), an inhibitor of reverse transcriptase (RTase). After a prolonged exposure of DSCs for 2 weeks, the population doubling time (PDT) was significantly (P < .05) increased by delayed cell growth in the DSCs treated with 250 and 500 μM NVP, compared with untreated DSCs. Furthermore, the G1 phase of cell cycle with high activity of senescence-associated β-galactosidase was also significantly (P < .05) increased in the 250 μM NVP-treated DSCs, compared with untreated DSCs. The level of telomerase activity was unchanged between control and treatment. However, following the treatment of NVP, negative surface markers for mesenchymal stem cells (MSCs), such as CD34 and CD45, were significantly (P < .05) increased, while positive surface markers for MSCs, such as CD90 and CD105, were significantly (P < .05) decreased in the NVP-treated DSCs than those of untreated DSCs. Furthermore, the differentiation capacity into mesodermal lineage was gradually decreased, and a significant (P < .05) decrease of expression level of NANOG, OCT-4 and SOX-2 transcripts was observed in the DSCs treated with NVP, compared with untreated control DSCs. Taken together, the present results have revealed that inhibition of RTase by NVP induces delayed cell growth and loss of stemness.

From molecular promise to preclinical results: HDAC inhibitors in the race for healthy aging drugs

Reversing or slowing the aging process brings great promise to treat or prevent age‐related disease, and targeting the hallmarks of aging is a strategy to achieve this. Epigenetics affects several if not all of the hallmarks of aging and has therefore emerged as a central target for intervention. One component of epigenetic regulation involves histone deacetylases (HDAC), which include the “classical” histone deacetylases (of class I, II, and IV) and sirtuin deacetylases (of class III). While targeting sirtuins for healthy aging has been extensively reviewed elsewhere, this review focuses on pharmacologically inhibiting the classical HDACs to promote health and longevity. We describe the theories of how classical HDAC inhibitors may operate to increase lifespan, supported by studies in model organisms. Furthermore, we explore potential mechanisms of how HDAC inhibitors may have such a strong grasp on health and longevity, summarizing their links to other hallmarks of aging. Finally, we show the wide range of age‐related preclinical disease models, ranging from neurodegeneration to heart disease, diabetes to sarcopenia, which show improvement upon HDAC inhibition.

Cellular senescence is associated with reorganization of the microtubule cytoskeleton

Senescent cells undergo structural and functional changes that affect essentially every aspect of cell physiology. To date, the impact of senescence on the cytoskeleton is poorly understood. This study evaluated the cytoskeleton in two independent cellular models of kidney epithelium senescence. Our work identified multiple senescence-related alterations that impact microtubules and filamentous actin during interphase. Both filamentous systems reorganized profoundly when cells became senescent. As such, microtubule stability increased during senescence, making these filaments more resistant to disassembly in the cold or by nocodazole. Microtubule stabilization was accompanied by enhanced α-tubulin acetylation on lysine 40 and the depletion of HDAC6, the major deacetylase for α-tubulin lysine 40. Rho-associated kinase Rock1 is an upstream regulator that modulates key properties of the cytoplasmic cytoskeleton. Our research shows that Rock1 concentrations were reduced significantly in senescent cells, and we revealed a mechanistic link between microtubule stabilization and Rock1 depletion. Thus, Rock1 overexpression partially restored the cold sensitivity of microtubules in cells undergoing senescence. Additional components relevant to microtubules were affected by senescence. Specifically, we uncovered the senescence-related loss of the microtubule nucleating protein γ-tubulin and aberrant formation of γ-tubulin foci. Concomitant with the alterations of microtubule and actin filaments, senescent cells displayed functional changes. In particular, cell migration was impaired significantly in senescent cells. Taken together, our study identified new senescence-associated deficiencies of the microtubule and actin cytoskeleton, provided insights into the underlying molecular mechanisms and demonstrated functional consequences that are important to the physiology and function of renal epithelial cells.

A pH probe inhibits senescence in mesenchymal stem cells

BACKGROUND

Bone marrow-derived mesenchymal stem cells (BMSCs) are gradually getting attention because of its multi-directional differentiation potential, hematopoietic support, and promotion of stem cell implantation. However, cultured BMSCs in vitro possess a very limited proliferation potential, and the presence of stem cell aging has substantially restricted the effect together with the efficiency in clinical treatment. Recently, increasing attention has been paid to the connection between cellular aging and lysosomal acidification as new reports indicated that vacuolar H⁺-ATPase (v-ATPase) activity was altered and lysosomal pH was dysregulated in the process of cellular aging. Therefore, promoting lysosomal acidification might contribute to inhibition of cell senescence. Our previous studies showed that a novel small molecule, 3-butyl-1-chloro imidazo [1, 5-a] pyridine-7-carboxylic acid (SGJ), could selectively and sensitively respond to acidic pH with fast response (within 3 min), but whether SGJ can promote lysosomal acidification and inhibit senescence in BMSCs is unknown.

METHODS

Rat BMSCs were cultured based on our system that had been already documented. BMSCs were treated with SGJ and/or Bafilomycin-A1 (Baf-A1). The co-localization between SGJ and lysosomes was assessed by a confocal microscope. Acridine orange (AO) staining and the Lysosensor™ Green DND-189 reagents were used for indicating changes in lysosomal concentration of H⁺. Changes of senescence were detected by immunoblotting of p21 and senescence-associated beta-galactosidase (SA-β-gal) staining as well as immunofluorescence assay of senescence-associated heterochromatin foci (SAHF). Changes of autophagy were detected by immunoblotting of MAP1LC3 (LC3B) and SQSTM1 (p62). Cell proliferation was determined by flow cytometry. Cell viability was calculated by sulforhodamine B assay (SRB). The V0 proton channel of v-ATPase was knocked down by transfecting with its small interfering RNA (si-ATP6V0C).

RESULTS

Our work showed that SGJ can promote lysosomal acidification and inhibit senescence in BMSCs. Firstly, SGJ and lysosomes were well co-located in senescent BMSCs with the co-localization coefficient of 0.94. Secondly, SGJ increased the concentration of H⁺ and the protein expression of lysosome-associated membrane protein 1 (LAMP1) and lysosome-associated membrane protein 2 (LAMP2). Thirdly, SGJ suppressed the expression of p21 in the senescent BMSCs and reduced SA-β-gal positive cells. Fourthly, SGJ promoted senescent BMSCs' proliferation and protein level of LC3B but reduced the p62/SQSTM1 protein level. Furthermore, experimental group pretreated with 20 μM SGJ showed a stronger red fluorescent intensity, thinner cell morphology, less SA-β-gal positive cell, and less p21 protein level as well as higher cell viability in the presence of Baf-A1. Notably, ATP6V0C knockdown decreased the activity of v-ATPase and SGJ increased the concentration of H⁺.

CONCLUSION

Our work showed that SGJ could inhibit senescence in BMSCs and protect lysosomes by promoting expression of LAMP1 and LAMP2. Meanwhile, SGJ could promote autophagy. Furthermore, our study also suggested that SGJ was a new Baf-A1 antagonist because SGJ could target and occupy the V0 proton channel of v-ATPase.

CK2 downregulation induces senescence-associated heterochromatic foci formation through activating SUV39h1 and inactivating G9a

Cellular senescence is an irreversible form of cell cycle arrest and senescent cells have a unique gene expression profile that is frequently accompanied by senescence-associated heterochromatic foci (SAHF). Here, we present evidence that CK2 downregulation induces trimethylation of histone H3 Lys 9 (H3K9me3), selective binding of HP1γ to H3K9me3, formation of SAHF, and reduction of cyclin D1 expression in HCT116 and MCF-7 cells. CK2 downregulation-mediated H3K9me3 is associated with induction of H3K9 trimethylase SUV39h1 as well as reduction of H3K9 dimethylase G9a and GLP in cells. In addition, Pharmacological inhibition of SUV39h1 and G9a overexpression significantly attenuated induction of senescence-associated β-galactosidase (SA-β-gal) activity, H3K9me3 and SAHF formation in CK2-downregulated cells. Moreover, CK2 downregulation induced H3K9me3 in nematodes. Taken together, these results demonstrate that CK2 downregulation leads to H3K9me3 and SAHF formation by increasing SUV39h1 and decreasing G9a.

A Heterochromatin Domain Forms Gradually at a New Telomere and Is Dynamic at Stable Telomeres

Heterochromatin domains play important roles in chromosome biology, organismal development, and aging, including centromere function, mammalian female X chromosome inactivation, and senescence-associated heterochromatin foci. In the fission yeast Schizosaccharomyces pombe and metazoans, heterochromatin contains histone H3 that is dimethylated at lysine 9.

Heterochromatin domains play important roles in chromosome biology, organismal development, and aging, including centromere function, mammalian female X chromosome inactivation, and senescence-associated heterochromatin foci. In the fission yeast Schizosaccharomyces pombe and metazoans, heterochromatin contains histone H3 that is dimethylated at lysine 9. While factors required for heterochromatin have been identified, the dynamics of heterochromatin formation are poorly understood. Telomeres convert adjacent chromatin into heterochromatin. To form a new heterochromatic region in S. pombe, an inducible DNA double-strand break (DSB) was engineered next to 48 bp of telomere repeats in euchromatin, which caused formation of a new telomere and the establishment and gradual spreading of a new heterochromatin domain. However, spreading was dynamic even after the telomere had reached its stable length, with reporter genes within the heterochromatin domain showing variegated expression. The system also revealed the presence of repeats located near the boundaries of euchromatin and heterochromatin that are oriented to allow the efficient healing of a euchromatic DSB to cap the chromosome end with a new telomere. Telomere formation in S. pombe therefore reveals novel aspects of heterochromatin dynamics and fail-safe mechanisms to repair subtelomeric breaks, with implications for similar processes in metazoan genomes.

Is there a role for placental senescence in the genesis of obstetric complications and fetal growth restriction?

The placenta ages as pregnancy advances, yet its continued function is required for a successful pregnancy outcome. Placental aging is a physiological phenomenon; however, there are some placentas that show signs of aging earlier than others. Premature placental senescence and aging are implicated in a number of adverse pregnancy outcomes, including fetal growth restriction, preeclampsia, spontaneous preterm birth, and intrauterine fetal death. Here we discuss cellular senescence, a state of terminal proliferation arrest, and how senescence is regulated. We also explore the role of physiological placental senescence and how aberrant placental senescence alters placental function, contributing to the pathophysiology of fetal growth restriction, preeclampsia, spontaneous preterm labor/birth, and unexplained fetal death.

Gene Expression, Epigenetics and Ageing

As the popular adage goes, all diseases run into old age and almost all physiological changes are associated with alterations in gene expression, irrespective of whether they are causal or consequential. Therefore, the quest for mechanisms that delay ageing and decrease age-associated diseases has propelled researchers to unravel regulatory factors that lead to changes in chromatin structure and function, which ultimately results in deregulated gene expression. It is therefore essential to bring together literature, which until recently has investigated gene expression and chromatin independently. With advances in biomedical research and the emergence of epigenetic regulators as potential therapeutic targets, enhancing our understanding of mechanisms that 'derail' transcription and identification of causal genes/pathways during ageing will have a significant impact. In this context, this chapter aims to not only summarize the key features of age-associated changes in epigenetics and transcription, but also identifies gaps in the field and proposes aspects that need to be investigated in the future.

Rhizoma Coptidis and Berberine as a Natural Drug to Combat Aging and Aging-Related Diseases via Anti-Oxidation and AMPK Activation

Aging is the greatest risk factor for human diseases, as it results in cellular growth arrest, impaired tissue function and metabolism, ultimately impacting life span. Two different mechanisms are thought to be primary causes of aging. One is cumulative DNA damage induced by a perpetuating cycle of oxidative stress; the other is nutrient-sensing adenosine monophosphate-activated protein kinase (AMPK) and rapamycin (mTOR)/ ribosomal protein S6 (rpS6) pathways. As the main bioactive component of natural Chinese medicine rhizoma coptidis (RC), berberine has recently been reported to expand life span in Drosophila melanogaster, and attenuate premature cellular senescence. Most components of RC including berberine, coptisine, palmatine, and jatrorrhizine have been found to have beneficial effects on hyperlipidemia, hyperglycemia and hypertension aging-related diseases. The mechanism of these effects involves multiple cellular kinase and signaling pathways, including anti-oxidation, activation of AMPK signaling and its downstream targets, including mTOR/rpS6, Sirtuin1/ forkhead box transcription factor O3 (FOXO3), nuclear factor erythroid-2 related factor-2 (Nrf2), nicotinamide adenine dinucleotide (NAD⁺) and nuclear factor-κB (NF-κB) pathways. Most of these mechanisms converge on AMPK regulation on mitochondrial oxidative stress. Therefore, such evidence supports the possibility that rhizoma coptidis, in particular berberine, is a promising anti-aging natural product, and has pharmaceutical potential in combating aging-related diseases via anti-oxidation and AMPK cellular kinase activation.

ATRX is a regulator of therapy induced senescence in human cells

Senescence is a state of stable cell cycle exit with important implications for development and disease. Here, we demonstrate that the chromatin remodeling enzyme ATRX is required for therapy-induced senescence. ATRX accumulates in nuclear foci and is required for therapy-induced senescence in multiple types of transformed cells exposed to either DNA damaging agents or CDK4 inhibitors. Mobilization into foci depends on the ability of ATRX to interact with H3K9me3 histone and HP1. Foci form soon after cells exit the cell cycle, before other hallmarks of senescence appear. Eliminating ATRX in senescent cells destabilizes the senescence-associated heterochromatic foci. Additionally, ATRX binds to and suppresses expression from the HRAS locus; repression of HRAS is sufficient to promote the transition of quiescent cells into senescence and preventing repression blocks progression into senescence. Thus ATRX is a critical regulator of therapy-induced senescence and acts in multiple ways to drive cells into this state.

Therapy induced senescence (TIS) is a growth suppressive program activated by cytostatic agents in some cancer cells. Here the authors show that the chromatin remodeling enzyme ATRX is a regulator of TIS and drives cells into this state via multiple mechanisms.

Induction of telomere shortening and cellular apoptosis by sodium meta-arsenite in human cancer cell lines

The present study assessed the cytotoxicity of sodium meta-arsenite (SMA) on telomere shortening and cellular apoptosis in human A-549, MDA-MB-231 and U87-MG cancer cell lines. Following 2 weeks of 1 μM SMA treatment, population doubling time (PDT) was significantly (P < .05) increased by the inhibition of cell proliferation in all the cancer cell lines compared to that in untreated controls. Level of telomerase activity by relative-quantitative telomerase repeat amplification protocol was significantly (P < .05) downregulated by SMA treatment with significant (P < .05) decrease of both telomerase reverse transcriptase and telomerase RNA component transcripts, responsible for telomerase activity. A significant (P < .05) shortening of telomeric repeats by telomere restriction fragment analysis was consequently observed in SMA-treated cells. Moreover, high incidence of cells with senescence-associated β-glucosidase activity was observed in SMA-treated cells and some cells were also differentiated into adipocytes probably due to the loss of tumorous characterizations. Cellular apoptosis proven by DNA fragmentation was observed, and intrinsic apoptotic transcripts (BAX, caspase 3 and caspase 9) and stress-related transcripts (p21, HSP70 and HSP90) were significantly (P < .05) increased in three cancer cell lines treated with SMA. Based on the present study, SMA treatment apparently induced a shortening of telomere length and cytotoxicity, such as induction of cell senescence, apoptosis and cell differentiation. Therefore, we conclude that SMA treatment at specific concentration can lead to gradual loss of tumorous characterizations and can be considered as a potential anti-cancer drug for chemotherapy treatment.

Shining the Light on Senescence Associated LncRNAs

Cellular senescence can be described as a complex stress response that leads to irreversible cell cycle arrest. This process was originally described as an event that primary cells go through after many passages of cells during cell culture. More recently, cellular senescence is viewed as a programmed process by which the cell displays a senescence phenotype when exposed to a variety of stresses. Cellular senescence has been implicated in tumor suppression and aging such that senescence may contribute to both tumor progression and normal tissue repair. Here, we review different forms of cellular senescence, as well as current biomarkers used to identify senescent cells in vitro and in vivo. Additionally, we highlight the role of senescence-associated long noncoding RNAs (lncRNAs).

A drive in SUVs: From development to disease

Progression of cells through distinct phases of the cell cycle, and transition into out-of-cycling states, such as terminal differentiation and senescence, is accompanied by specific patterns of gene expression. These cell fate decisions are mediated not only by distinct transcription factors, but also chromatin modifiers that establish heritable epigenetic patterns. Lysine methyltransferases (KMTs) that mediate methylation marks on histone and non-histone proteins are now recognized as important regulators of gene expression in cycling and non-cycling cells. Among these, the SUV39 sub-family of KMTs, which includes SUV39H1, SUV39H2, G9a, GLP, SETDB1, and SETDB2, play a prominent role. In this review, we discuss their biochemical properties, sub-cellular localization and function in cell cycle, differentiation programs, and cellular senescence. We also discuss their aberrant expression in cancers, which exhibit de-regulation of cell cycle and differentiation.

Roles of pRB in the Regulation of Nucleosome and Chromatin Structures

Retinoblastoma protein (pRB) interacts with E2F and other protein factors to play a pivotal role in regulating the expression of target genes that induce cell cycle arrest, apoptosis, and differentiation. pRB controls the local promoter activity and has the ability to change the structure of nucleosomes and/or chromosomes via histone modification, epigenetic changes, chromatin remodeling, and chromosome organization. Functional inactivation of pRB perturbs these cellular events and causes dysregulated cell growth and chromosome instability, which are hallmarks of cancer cells. The role of pRB in regulation of nucleosome/chromatin structures has been shown to link to tumor suppression. This review focuses on the ability of pRB to control nucleosome/chromatin structures via physical interactions with histone modifiers and chromatin factors and describes cancer therapies based on targeting these protein factors.

Physiological and Pathological Aging Affects Chromatin Dynamics, Structure and Function at the Nuclear Edge

Lamins are intermediate filaments that form a complex meshwork at the inner nuclear membrane. Mammalian cells express two types of Lamins, Lamins A/C and Lamins B, encoded by three different genes, LMNA, LMNB1, and LMNB2. Mutations in the LMNA gene are associated with a group of phenotypically diverse diseases referred to as laminopathies. Lamins interact with a large number of binding partners including proteins of the nuclear envelope but also chromatin-associated factors. Lamins not only constitute a scaffold for nuclear shape, rigidity and resistance to stress but also contribute to the organization of chromatin and chromosomal domains. We will discuss here the impact of A-type Lamins loss on alterations of chromatin organization and formation of chromatin domains and how disorganization of the lamina contributes to the patho-physiology of premature aging syndromes.

Homologous recombination-dependent repair of telomeric DSBs in proliferating human cells

Telomeres prevent chromosome ends from being recognized as double-stranded breaks (DSBs). Meanwhile, G/C-rich repetitive telomeric DNA is susceptible to attack by DNA-damaging agents. How cells balance the need to protect DNA ends and the need to repair DNA lesions in telomeres is unknown. Here we show that telomeric DSBs are efficiently repaired in proliferating cells, but are irreparable in stress-induced and replicatively senescent cells. Using the CRISPR-Cas9 technique, we specifically induce DSBs at telomeric or subtelomeric regions. We find that DSB repair (DSBR) at subtelomeres occurs in an error-prone manner resulting in small deletions, suggestive of NHEJ. However, DSBR in telomeres involves 'telomere-clustering', 3'-protruding C-rich telomeric ssDNA, and HR between sister-chromatid or interchromosomal telomeres. DSBR in telomeres is suppressed by deletion or inhibition of Rad51. These findings reveal proliferation-dependent DSBR in telomeres and suggest that telomeric HR, which is normally constitutively suppressed, is activated in the context of DSBR.

The Metabolic Impact on Histone Acetylation and Transcription in Ageing

Loss of cellular homeostasis during aging results in altered tissue functions and leads to a general decline in fitness and, ultimately, death. As animals age, the control of gene expression, which is orchestrated by multiple epigenetic factors, degenerates. In parallel, metabolic activity and mitochondrial protein acetylation levels also change. These two hallmarks of aging are effectively linked through the accumulating evidence that histone acetylation patterns are susceptible to alterations in key metabolites such as acetyl-CoA and NAD(+), allowing chromatin to function as a sensor of cellular metabolism. In this review we discuss experimental data supporting these connections and provide a context for the possible medical and physiological relevance.

Hypoxia-Inducible Histone Lysine Demethylases: Impact on the Aging Process and Age-Related Diseases

Hypoxia is an environmental stress at high altitude and underground conditions but it is also present in many chronic age-related diseases, where blood flow into tissues is impaired. The oxygen-sensing system stimulates gene expression protecting tissues against hypoxic insults. Hypoxia stabilizes the expression of hypoxia-inducible transcription factor-1α (HIF-1α), which controls the expression of hundreds of survival genes related to e.g. enhanced energy metabolism and autophagy. Moreover, many stress-related signaling mechanisms, such as oxidative stress and energy metabolic disturbances, as well as the signaling cascades via ceramide, mTOR, NF-κB, and TGF-β pathways, can also induce the expression of HIF-1α protein to facilitate cell survival in normoxia. Hypoxia is linked to prominent epigenetic changes in chromatin landscape. Screening studies have indicated that the stabilization of HIF-1α increases the expression of distinct histone lysine demethylases (KDM). HIF-1α stimulates the expression of KDM3A, KDM4B, KDM4C, and KDM6B, which enhance gene transcription by demethylating H3K9 and H3K27 sites (repressive epigenetic marks). In addition, HIF-1α induces the expression of KDM2B and KDM5B, which repress transcription by demethylating H3K4me2,3 sites (activating marks). Hypoxia-inducible KDMs support locally the gene transcription induced by HIF-1α, although they can also control genome-wide chromatin landscape, especially KDMs which demethylate H3K9 and H3K27 sites. These epigenetic marks have important role in the control of heterochromatin segments and 3D folding of chromosomes, as well as the genetic loci regulating cell type commitment, proliferation, and cellular senescence, e.g. the INK4 box. A chronic stimulation of HIF-1α can provoke tissue fibrosis and cellular senescence, which both are increasingly present with aging and age-related diseases. We will review the regulation of HIF-1α-dependent induction of KDMs and clarify their role in pathological processes emphasizing that long-term stress-related insults can impair the maintenance of chromatin landscape and provoke cellular senescence and tissue fibrosis associated with aging and age-related diseases.

Histone demethylase JMJD3 at the intersection of cellular senescence and cancer

Cellular senescence is defined by an irreversible growth arrest and is an important biological mechanism for suppression of tumor formation. Although deletion/mutation to DNA sequences is one mechanism by which cancer cells can escape senescence, little is known about the epigenetic factors contributing to this process. Histone modifications and chromatin remodeling related to the function of a histone demethylase, jumonji domain-containing protein 3 (JMJD3; also known as KDM6B), play an important role in development, tissue regeneration, stem cells, inflammation, and cellular senescence and aging. The role of JMJD3 in cancer is poorly understood and its function may be at the intersection of many pathways promoted in a dysfunctional manner such as activation of the senescence-associated secretory phenotype (SASP) observed in aging.

Nrf2 signaling and redox homeostasis in the aging heart: A potential target to prevent cardiovascular diseases?

Aging process is often accompanied with a high incidence of cardiovascular diseases (CVD) due to the synergistic effects of age-related changes in heart morphology/function and prolonged exposure to injurious effects of CVD risk factors. Oxidative stress, considered a hallmark of aging, is also an important feature in pathologies that predispose to CVD development, like hypertension, diabetes and obesity. Approaches directed to prevent the occurrence of CVD during aging have been explored both in experimental models and in controlled clinical trials, in order to improve health span, reduce hospitalizations and increase life quality during elderly. In this review we discuss oxidative stress role as a main risk factor that relates CVD with aging. As well as interventions that aim to reduce oxidative stress by supplementing with exogenous antioxidants. In particular, strategies of improving the endogenous antioxidant defenses through activating the nuclear factor related-2 factor (Nrf2) pathway; one of the best studied molecules in cellular redox homeostasis and a master regulator of the antioxidant and phase II detoxification response.

Retinoblastoma-binding Protein 4-regulated Classical Nuclear Transport Is Involved in Cellular Senescence

Nucleocytoplasmic trafficking is a fundamental cellular process in eukaryotic cells. Here, we demonstrated that retinoblastoma-binding protein 4 (RBBP4) functions as a novel regulatory factor to increase the efficiency of importin α/β-mediated nuclear import. RBBP4 accelerates the release of importin β1 from importin α via competitive binding to the importin β-binding domain of importin α in the presence of RanGTP. Therefore, it facilitates importin α/β-mediated nuclear import. We showed that the importin α/β pathway is down-regulated in replicative senescent cells, concomitant with a decrease in RBBP4 level. Knockdown of RBBP4 caused both suppression of nuclear transport and induction of cellular senescence. This is the first report to identify a factor that competes with importin β1 to bind to importin α, and it demonstrates that the loss of this factor can trigger cellular senescence.

Analysis of gene expression during aging of CGNs in culture: implication of SLIT2 and NPY in senescence

Senescence is the major key factor that leads to the loss of neurons throughout aging. Cellular senescence is not the consequence of single cause, but there are multiple aspects which may induce senescence in a cell. Various causes such as gene expression, molecular interactions and protein processing and chromatin organization are described as causal factor for senescence. It is well known that the damage to the nuclear or mitochondrial DNA contributes to the aging either directly by inducing the apoptosis/cellular senescence or indirectly by altering cellular functions. The significant nuclear DNA damage with the age is directly associated with the continuous declining in DNA repair. The continuous decline in expression of topoisomerase 2 beta (Topo IIβ) in cultured cerebellar granule neurons over time indicated the decline in the repair of damage DNA. DNA Topo IIβ is an enzyme that is crucial for solving topological problems of DNA and thus has an important role in DNA repair. The enzyme is predominantly present in non-proliferating cells such as neurons. In this paper, we have studied the genes which were differentially expressed over time in cultured cerebellar granule neurons (CGNs) and identified potential genes associated with the senescence. Our results showed that the two genes neuropeptide Y (Npy) and Slit homolog 2 (Drosophila) (Slit2) gradually increase during aging, and upon suppression of these two genes, there was gradual increase in cell viability along with restoration of the expression of Topo IIβ and potential repair proteins.

The emerging role of senescent cells in tissue homeostasis and pathophysiology

Cellular senescence is a state of permanent growth arrest and is thought to play a pivotal role in tumor suppression. Cellular senescence may play an important role in tumor suppression, wound healing, and protection against tissue fibrosis in physiological conditions in vivo. However, accumulating evidence that senescent cells may have harmful effects in vivo and may contribute to tissue remodeling, organismal aging, and many age-related diseases also exists. Cellular senescence can be induced by various intrinsic and extrinsic factors. Both p53/p21 and p16/RB pathways are important for irreversible growth arrest in senescent cells. Senescent cells secret numerous biologically active factors. This specific secretion phenotype by senescent cells may largely contribute to physiological and pathological consequences in organisms. Here I review the molecular basis of cell cycle arrest and the specific secretion phenotype in cellular senescence. I also summarize the current knowledge of the role of cellular senescence in vivo in physiological and pathological settings.

Wip1 knockout inhibits the proliferation and enhances the migration of bone marrow mesenchymal stem cells

Mesenchymal stem cells (MSCs), a unique population of multipotent adult progenitor cells originally found in bone marrow (BM), are extremely useful for multifunctional therapeutic approaches. However, the growth arrest and premature senescence of MSCs in vitro prevent the in-depth characterization of these cells. In addition, the regulatory factors involved in MSCs migration remain largely unknown. Given that protein phosphorylation is associated with the processes of MSCs proliferation and migration, we focused on wild-type p53-inducible phosphatase-1 (Wip1), a well-studied modulator of phosphorylation, in this study. Our results showed that Wip1 knockout significantly inhibited MSCs proliferation and induced G2-phase cell-cycle arrest by reducing cyclinB1 expression. Compared with WT-MSCs, Wip1(-/-) MSCs displayed premature growth arrest after six passages in culture. Transwell and scratch assays revealed that Wip1(-/-) MSCs migrate more effectively than WT-MSCs. Moreover, the enhanced migratory response of Wip1(-/-) MSCs may be attributed to increases in the induction of Rac1-GTP activity, the pAKT/AKT ratio, the rearrangement of filamentous-actin (f-actin), and filopodia formation. Based on these results, we then examined the effect of treatment with a PI3K/AKT and Rac1 inhibitor, both of which impaired the migratory activity of MSCs. Therefore, we propose that the PI3K/AKT/Rac1 signaling axis mediates the Wip1 knockout-induced migration of MSCs. Our findings indicate that the principal function of Wip1 in MSCs transformation is the maintenance of proliferative capacity. Nevertheless, knocking out Wip1 increases the migratory capacity of MSCs. This dual effect of Wip1 provides the potential for purposeful routing of MSCs.

Long-term arsenite exposure induces premature senescence in B cell lymphoma A20 cells

Chronic arsenite exposure induces immunosuppression, but the precise mechanisms remain elusive. Our previous studies demonstrated that arsenite exposure for 24 h induces G0/G1 arrest in mouse B lymphoma A20 cells and the arrest is caused through induction of cyclin-dependent kinase inhibitor p16(INK4a) followed by accumulation of an Rb family protein, p130. In this study, we further investigated the consequences of long-term arsenite exposure of A20 cells. The results demonstrated that exposure to 10 μM sodium arsenite up to 14 days induces a great increase in G0/G1 arrest, irreversible cell growth suppression, cellular morphological changes and positive staining for senescence-associated β-galactosidase. The long-term arsenite exposure also induced up-regulation of p16(INK4a) followed by robust accumulation of p130 and activation of the p53 pathway. Knockdown experiments with siRNA showed that p130 accumulation is essential for cell cycle arrest by long-term arsenite exposure. Since p16(INK4a) and the p53 pathway are known to be activated by DNA damage, we investigated the involvement of DNA damage formation by long-term arsenite exposure. We found that a variety of DNA repair-related genes were significantly down-regulated from 24 h of arsenite exposure and activation-induced cytidine deaminase was greatly up-regulated after long-term arsenite exposure. Consistent with these findings, long-term arsenite exposure increased a DNA double-strand break marker, γ-H2AX and increased mutation frequency in a Bcl6 gene region. These results revealed that long-term arsenite exposure induces premature senescence through DNA damage increase and p130 accumulation in lymphoid cells.

Senescent human fibroblasts show increased glycolysis and redox homeostasis with extracellular metabolomes that overlap with those of irreparable DNA damage, aging, and disease

Cellular senescence can modulate various pathologies and is associated with irreparable DNA double-strand breaks (IrrDSBs). Extracellular senescence metabolomes (ESMs) were generated from fibroblasts rendered senescent by proliferative exhaustion (PEsen) or 20 Gy of γ rays (IrrDSBsen) and compared with those of young proliferating cells, confluent cells, quiescent cells, and cells exposed to repairable levels of DNA damage to identify novel noninvasive markers of senescent cells. ESMs of PEsen and IrrDSBsen overlapped and showed increased levels of citrate, molecules involved in oxidative stress, a sterol, monohydroxylipids, tryptophan metabolism, phospholipid, and nucleotide catabolism, as well as reduced levels of dipeptides containing branched chain amino acids. The ESM overlaps with the aging and disease body fluid metabolomes, supporting their utility in the noninvasive detection of human senescent cells in vivo and by implication the detection of a variety of human pathologies. Intracellular metabolites of senescent cells showed a relative increase in glycolysis, gluconeogenesis, the pentose-phosphate pathway, and, consistent with this, pyruvate dehydrogenase kinase transcripts. In contrast, tricarboxylic acid cycle enzyme transcript levels were unchanged and their metabolites were depleted. These results are surprising because glycolysis antagonizes senescence entry but are consistent with established senescent cells entering a state of low oxidative stress.

The histone demethylase jumonji coordinates cellular senescence including secretion of neural stem cell-attracting cytokines

Jumonji domain-containing protein 3 (JMJD3/KDM6B) demethylates lysine 27 on histone H3 (H3K27me3), a repressive epigenetic mark controlling chromatin organization and cellular senescence. To better understand the functional consequences of JMJD3 its expression was investigated in brain tumor cells. Querying patient expression profile databases confirmed JMJD3 overexpression in high-grade glioma. Immunochemical staining of two glioma cell lines, U251 and U87, indicated intrinsic differences in JMJD3 expression levels that were reflected in changes in cell phenotype and variations associated with cellular senescence, including senescence-associated β-galactosidase (SA-β-gal) activity and the senescence-associated secretory phenotype (SASP). Overexpressing wild-type JMJD3 (JMJD3wt) activated SASP-associated genes, enhanced SA-β-gal activity, and induced nuclear blebbing. Conversely, overexpression of a catalytically inactive dominant negative mutant JMJD3 (JMJD3mut) increased proliferation. In addition, a large number of transcripts were identified by RNA-seq as altered in JMJD3 overexpressing cells, including cancer- and inflammation-related transcripts as defined by Ingenuity Pathway Analysis. These results suggest that expression of the SASP in the context of cancer undermines normal tissue homeostasis and contributes to tumorigenesis and tumor progression. These studies are therapeutically relevant because inflammatory cytokines have been linked to homing of neural stem cells and other stem cells to tumor loci.

IMPLICATIONS

This glioma study brings together actions of a normal epigenetic mechanism (JMJD3 activity) with dysfunctional activation of senescence-related processes, including secretion of SASP proinflammatory cytokines and stem cell tropism toward tumors.

The Intricate Interplay between Mechanisms Underlying Aging and Cancer

Age is the major risk factor in the incidence of cancer, a hyperplastic disease associated with aging. Here, we discuss the complex interplay between mechanisms underlying aging and cancer as a reciprocal relationship. This relationship progresses with organismal age, follows the history of cell proliferation and senescence, is driven by common or antagonistic causes underlying aging and cancer in an age-dependent fashion, and is maintained via age-related convergent and divergent mechanisms. We summarize our knowledge of these mechanisms, outline the most important unanswered questions and suggest directions for future research.

DNA double strand break responses and chromatin alterations within the aging cell

Cellular senescence is a state of permanent replicative arrest that allows cells to stay viable and metabolically active but resistant to apoptotic and mitogenic stimuli. Specific, validated markers can identify senescent cells, including senescence-associated β galactosidase activity, chromatin alterations, cell morphology changes, activated p16- and p53-dependent signaling and permanent cell cycle arrest. Senescence is a natural consequence of DNA replication-associated telomere erosion, but can also be induced prematurely by telomere-independent events such as failure to repair DNA double strand breaks. Here, we review the molecular pathways of senescence onset, focussing on the changes in chromatin organization that are associated with cellular senescence, particularly senescence-associated heterochromatin foci formation. We also discuss the altered dynamics of the DNA double strand break response within the context of aging cells. Appreciating how, mechanistically, cellular senescence is induced, and how changes to chromatin organization and DNA repair contributes to this, is fundamental to our understanding of the normal and premature human aging processes associated with loss of organ and tissue function in humans.

Attenuation of replication stress-induced premature cellular senescence to assess anti-aging modalities

Described is an in vitro model of premature senescence in pulmonary adenocarcinoma A549 cells induced by persistent DNA replication stress in response to treatment with the DNA damaging drug mitoxantrone (Mxt). The degree of cellular senescence, based on characteristic changes in cell morphology, is measured by laser scanning cytometry. Specifically, the flattening of cells grown on slides (considered the hallmark of cellular senescence) is measured as the decline in local intensity of DNA-associated DAPI fluorescence (represented by maximal pixels). This change is paralleled by an increase in nuclear area. Thus, the ratio of mean intensity of maximal pixels to nuclear area provides a very sensitive morphometric biomarker for the degree of senescence. This analysis is combined with immunocytochemical detection of senescence markers, such as overexpression of cyclin kinase inhibitors (e.g., p21(WAF1) ) and phosphorylation of ribosomal protein S6 (rpS6), a key marker associated with aging/senescence that is detected using a phospho-specific antibody. These biomarker indices are presented in quantitative terms defined as a senescence index (SI), which is the fraction of the marker in test cultures relative to the same marker in exponentially growing control cultures. This system can be used to evaluate the anti-aging potential of test agents by assessing attenuation of maximal senescence. As an example, the inclusion of berberine, a natural alkaloid with reported anti-aging properties and a long history of use in traditional Chinese medicine, is shown to markedly attenuate the Mxt-induced SI and phosphorylation of rpS6. The multivariate analysis of senescence markers by laser scanning cytometry offers a promising tool to explore the potential anti-aging properties of a variety agents.

Chromatin maintenance and dynamics in senescence: a spotlight on SAHF formation and the epigenome of senescent cells

Senescence is a stable proliferation arrest characterized by profound changes in cellular morphology and metabolism as well as by extensive chromatin reorganization in the nucleus. One particular hallmark of chromatin changes during senescence is the formation of punctate DNA foci in DAPI-stained senescent cells that have been called senescence-associated heterochromatin foci (SAHF). While many advances have been made concerning our understanding of the effectors of senescence, how chromatin is reorganized and maintained in senescent cells has remained largely elusive. Because chromatin structure is inherently dynamic, senescent cells face the challenge of developing chromatin maintenance mechanisms in the absence of DNA replication in order to maintain the senescent phenotype. Here, we summarize and review recent findings shedding light on SAHF composition and formation via spatial repositioning of chromatin, with a specific focus on the role of lamin B1 for this process. In addition, we discuss the physiological implication of SAHF formation, the role of histone variants, and histone chaperones during senescence and also elaborate on the more general changes observed in the epigenome of the senescent cells.

p19INK4d is involved in the cellular senescence mechanism contributing to heterochromatin formation

BACKGROUND

During evolution, organisms with renewable tissues have developed mechanisms to prevent tumorigenesis, including cellular senescence and apoptosis. Cellular senescence is characterized by a permanent cell cycle arrest triggered by both endogenous stress and exogenous stress. The p19INK4d, a member of the family of cyclin-dependent kinase inhibitors (INK4), plays an important role on cell cycle regulation and in the cellular DNA damage response. We hypothesize that p19INK4d is a potential factor involved in the onset and/or maintenance of the senescent state.

METHODS

Senescence was confirmed by measuring the cell cycle arrest and the senescence-associated β-galactosidase activity. Changes in p19INK4d expression and localization during senescence were determined by Western blot and immunofluorescence assays. Chromatin condensation was measured by microccocal nuclease digestion and histone salt extraction.

RESULTS

The data presented here show for the first time that p19INK4d expression is up-regulated by different types of senescence. Changes in senescence-associated hallmarks were driven by modulation of p19 expression indicating a direct link between p19INK4d induction and the establishment of cellular senescence. Following a senescence stimulus, p19INK4d translocates to the nucleus and tightly associates with chromatin. Moreover, reduced levels of p19INK4d impair senescence-related global genomic heterochromatinization. Analysis of p19INK4d mRNA and protein levels in tissues from differently aged mice revealed an up-regulation of p19INK4d that correlates with age.

CONCLUSION

We propose that p19INK4d participates in the cellular mechanisms that trigger senescence by contributing to chromatin compaction.

GENERAL SIGNIFICANCE

This study provides novel insights into the dynamics process of cellular senescence, a central tumor suppressive mechanism.